This invention relates to a process for making a product having selective metal pathways which can function as an electrically conductive circuit.
Prior to the present invention, a wide variety of processes has been available for forming electrically conductive pathways on a non-electrically conducting substrate. For example, a metal film such as a film formed from copper can be applied to a non-conducting substrate to form a resultant laminate in a first step. Typical substrates include rigid composites of fiber glass and polyester or fiber glass and epoxy or plastic films of polyester or polyimide. In a commonly employed process, a layer of conductive metal such as copper is laminated, typically with the use of an adhesive layer, to the non-conductive substrate in a first step. A photoactivatible resin composition (photoresist) then is coated on the metal layer and is subsequently exposed to a light pattern using a light mask to reproduce the original metal pathway pattern desired followed by photoresist development. Metal etching then is effected in the area unprotected by the resist, thereby to produce the desired pattern. Alternatively, an etch resist can be directly printed such as by silk screen, gravure or the like on the metal laminate sheet followed by curing and metal etching. This multistep process is slow and time-consuming and utilizes an expensive metal laminated substrate such as a copper laminated substrate where much of the copper is etched off and subsequently wasted.
An alternative presently available process comprises direct circuit printing on a substrate utilizing a conductive metal-filled thick film ink or printing paste. In these processes, the conductive inks are limited to either very expensive silver or gold precious metals since they have the requisite high conductivity combined with resistance to oxidation. Attempts to utilize these conductive inks with less expensive nickel, copper or aluminum powder, for example, have proven to be unsatisfactory for many applications since these metals, in the form of fine powders, do not provide the requisite conductivity primarily due to surface oxidation. The less expensive and highly conductive copper metal is difficult to use in such a direct printing metal ink due to the rapid surface oxidation of the copper when in the form of fine powder such as irregular particles, spheres or flakes. Thus, this process is not amenable to manufacturing economical highly conducting copper circuitry patterns.
There are also highly specialized processes involving metal filled inks which have not found wide utility due to the difficulty in controlling the process. Printed inks containing nickel and zinc powder have been utilized in an augmentative replacement process to make conductive copper patterns once exposed to an acidic copper sulfate solution (U.S. Pat. Nos. 4,404,237 and 4,470,883). These inks will form conductive patterns, however, with insufficient conductivity for many printed circuit applications as the copper is interpenetrated with the ink's binding resin. As disclosed in U.S. Pat. No. 4,327,124, copper powder has been rolled onto a screen printed copper-containing ink to provide conductive patterns by the wire ink process. The conductive patterns are highly brittle due to the metal powder content and do not adhere well to common printed circuit substrates. Additionally, electron curing of screen printed inks has been developed. Both the wire ink process and the electron curing of screen printed inks have met with limited success primarily due to the extensive equipment needed which is not readily available. None of the above compositions are photoimagable and when screen printed have line resolution capability only of 10-15 mils.
In other processes, coatings containing noble or nonnoble metals or metal salts dispersed as fine particles, usually in a polymeric binder, function as seed sites for subsequent plating with a metal. The polymeric compositions containing the metal or metal salt is applied to a substrate in the desired pattern. After being applied or printed, the composition is heat cured in order to drive off solvent and to cross-link the polymer. The high temperatures, e.g. 160.degree. C., and extended cure times, e.g., 1-2 hours, required for these products limits their use to products having good high temperature stability where the products are made by batch processes. Palladium is a typical activating material and is an expensive raw material that is only partially utilized since only the surface metal and the metal immediately adjacent to the surface is actually used to initiate plating. All the remaining buried metal is not utilized and, therefore, constitutes an unnecessary expense. In addition to this expense of metal overburden, the availability and cost of precious/semiprecious metal is volatile. Furthermore, printed lines only about 5 mils or greater are possible so that electrically conductive circuitry produced thereby is highly limited. Finally, the presence of metallic catalyst within the patterned resin carrier will undesirably dissipate electrical current away from the conductive path coated on the patterned cured resin carrier during use of the circuit. This greatly reduces the efficiency of the circuit by raising power requirements and by increasing the probability of short circuiting during use. Typical examples of these processes are disclosed, for example, in U.S. Pat. Nos. 3,900,320, 3,775,176 and 3,600,330.
It has also been proposed to form electrically conductive metal pathways by a process which includes coating a substrate with a composition containing a reducible metal complex. In one such process, the photoforming process, a substrate is coated with a sorbitol copper formate solution containing a photoactivated reducing agent. Upon exposure to ultraviolet radiation, unmasked areas are reduced to copper metal and are suitable for plating nucleation sites. Non-exposed areas are washed clean and all copper formate is removed before plating can be carried out. Although the photo-reducible copper solution is readily applied and selectively ultraviolet radiation treated, plating selectivity becomes a severe problem due to the high incidence of copper formate remaining on non-desired areas of the substrate. This results in a low reliability of the photo-selective copper reduction process. Examples of this technology are set forth in U.S. Pat. Nos. 4,268,536, 4,181,750, 4,133,908, 4,192,764, 4,167,601 and 3,925,578.
Argon laser reduction of silver nitrate to silver in a polyamic acid matrix has been developed as a technique for forming fine metal lines as disclosed in U.S. Pat. No. 4,526,807. An additive method for producing patterns on glass is claimed utilizing a silver-containing photographic emulsion coated onto the entire glass surface. After exposure, development and burning off of gelatin, a silver pattern remains which can then be electrolessly plated (Ger.(East) DD No. 213,311). These processes have the same inherent difficulties as the photoforming process. Specifically, plating selectivity is difficult to achieve due to the presence of silver in non-desired areas of the substrate.
It has also been proposed to apply to a substrate nonmetal containing coatings which are subsequently activated for plating by surface treatment with noble and/or non-noble metals or metal salts. Typical examples of these processes are shown in U.S Pat. Nos. 4,089,993, 4,073,981, 4,100,037 and 4,006,047. All of the coated substrates are heat cured and require elevated temperatures for long cure times. Examples of utilizing nonnoble metals such as copper, nickel or cobalt are shown in U.S. Pat. Nos. 4,006,047, 4,077,853 and 4,234,628. These systems all require the interaction of at least two non-noble metals for rendering a surface receptive to plating and are limited to complete plating coverage of an article and are not utilized in selective printing and plating to form electrically conductive pathways.
Selective additive plating on poly(phenylene sulfide) film is claimed by selectively doping non-laser-annealed areas of the film with iodine or arsenic pentafluoride. The doped areas are then not susceptible to electroless plating while the non-doped, but annealed, areas will serve as a template for electroless metal deposition. (U.S. Pat. No. 4,486,463). This process is unique to substrates such as poly(phenylene sulfide) which can be selectively doped to form semi-conductive pathways which then resist electroless plating.
The above-described plating art is utilized primarily to form electrically conducting metal pathways which function as electrical circuits in a wide variety of applications such as printed circuit boards using a relatively rigid base or so-called flexcircuits using a flexible plastic base where the circuit is sandwiched between two layers of flexible plastic. Typical bases for printed circuit boards include epoxy-fiber glass composites or phenolic-fiber glass composites. Typical bases for flexcircuits include polyimide and polyester. Polyimide, such as Kapton.RTM., is generally preferred for many applications because of high temperature stability needed for solder connections and service life. In forming such flexcircuits, the copper is laminated to the flexible polymer base, and then selectively etched to form the desired circuit. The printed circuit boards now utilized can be single sided, double sided or multilayered wherein electrically conductive paths are sandwiched between dielectric layers. The dielectric layer can be the rigid or flexible substrate of a resist. Thus, in known subtractive processes, the polymer base must be capable of withstanding the etching composition so that it is not degraded during circuit formation.
The above-described plating processes also can be utilized to form thin flexible resonant circuits which are useful for electronic security and article theft detection systems. While these circuits are electrically conductive, in use they are passive in that they are not used primarily as current carrying devices. Article theft detection systems are known in which electromagnetic waves are generated at a checkpoint and are caused to sweep repetitively at a given rate through a predetermined frequency range. A resonant electrical circuit tag is affixed to articles to be protected, the electrical circuit being resonant at a frequency within the swept frequency range. Changes in energy level which occur at a specific frequency within the swept repeating frequency band are detected, indicating the presence of the tag in the field. The electrical circuit comprises a coil and a capacitor connected to form a resonant loop. The tag circuit comprises an insulative substrate having one portion of the circuit formed on the opposite side of the substrate. Electrical connection is made between the portions of the circuit on opposite sides of the substrate by means of a conductive pin or eyelet extending through the substrate, or by means of a spot weld joining confronting circuit areas as disclosed, for example, by U.S. Pat. Nos. 3,863,244, 3,967,161, 4,021,705 and 4,369,557.
It would be highly desirable to provide a simpler, less costly and more rapid process for forming electrically conductive pathways on any one of a variety of non-conducting substrates. It would be desirable to provide such a process which eliminates the need for any metal etching step and which does not require the use of high temperatures or extensive curing time. By providing such a process, a wide variety of substrates, including thin film substrates or rigid substrates, could be utilized to support electrically conductive paths. Furthermore, it would be desirable to provide such a process for making very fine or thin electrically conductive pathways and a process whereby a dielectric, non-metal containing, resin is utilized. Such composite products could be utilized to produce metallized patterns such as those utilized on printed circuit boards, multilayer boards, electronic article surveillance circuit constructions utilized in article theft detection systems, decorative articles or the like.